Conventionally, an injection device includes a plurality of injectors, each of the injectors being controlled to open and close by an electronic control means, by means of control signals permitting driving of one or more pilot injections and a main injection at each of the injectors. The injectors used may be of several types, for example of solenoid type or piezoelectric type.
The document EP 0 740 068 describes a solenoid injector. The injector includes an injector body. At its lower end, the injector body defines a seat in which the lower end of a needle is able to engage, the needle being able to slide between an open position in which it permits ejection of fuel from the injector and a closed position in which it sealedly closes the injector. The injector body is supplied with fuel by a high-pressure fuel source, such as a common rail, using a supply passage emerging in an annular gallery. The annular gallery surrounds the needle, in the proximity of its upper end, the form of the needle being suitable to allow circulation of fuel from the annular gallery between the bore and the needle. The high pressure supply line also communicates with a control chamber through a “restrictor”. At its upper end, the control chamber is closed by a plate. The plate co-operates with a sliding valve member including a hollow rod, the inside of the hollow rod being able to communicate with the inside of the chamber when the valve member is disengaged from the plate. The inside of the hollow rod also communicates with a low-pressure return. An electronic control means controls, by means of control signals, a solenoid actuator. When the solenoid is supplied, the valve member is disengaged from the plate. At this moment, fuel in the control chamber can escape to the inside of the hollow rod and then into the low-pressure return. When the pressure inside the control chamber drops to a certain point, the force applied to the needle due to the pressure inside the control chamber is no longer sufficient to keep the needle in its closed position. At this moment, the needle adopts its open position and fuel is ejected from the injector. When the solenoid is no longer supplied, the valve member is re-engaged in the plate under the influence of a spring. This has the effect of closing communication between the inside of the hollow rod and the control chamber. At this moment, the pressure in the control chamber increases and pushes the needle into its closed position.
The document EP 0 937 891 describes a piezoelectric injector. The injector includes a piston which defines a control chamber in combination with the upper surface of the needle. The injector includes piezoelectric actuators. The actuators are electrically connected to a control circuit able to emit control signals. The pressurised fuel present in the control chamber applies a force to the upper part of the needle and allows it to be held in the closed position, in combination with a spring. To start the injection, the piezoelectric material is discharged, in order to reduce its size. The effect of this is a movement of the piston in the direction opposite to the needle and therefore a reduction in the pressure inside the control chamber. At this moment, the needle is in its open position. When the piezoelectric material is charged, this has the effect of pushing the piston downwardly. This movement increases the fuel pressure inside the control chamber, thus increasing the force applied to the upper surface of the needle, which has the effect of pushing it back into its closed position.
Even if the injectors used in the injection device are of the same type, each injector has specific parameters. In addition, mechanical wear can also affect the accuracy of the quantity of fuel injected. Learning processes must therefore be performed to adapt the control signals to the specific characteristics of each of the injectors, in order to balance the operation of the engine to the maximum extent, to optimise combustion noise and control gas emissions. In particular, these processes permit determination for each injector of the minimum drive pulse (MDP) causing opening of the injector.
A first solution consists of using an accelerometer. However, this solution is sensitive to vibrations and this causes accuracy problems, particularly with piezoelectric injectors.
A second solution consists of using a velocity sensor permitting continuous determination of the crankshaft velocity.
The document FR 2 720 787 describes a process for determination of the specific parameters of each of the injectors of an injection device of a combustion engine, in particular of a device with pilot injection and main injection. For this purpose, the curve is determined of the difference in instantaneous velocity of the drive shaft between the instant of passage through combustion top dead centre of the cylinder in question and a subsequent predetermined instant, for example offset by 60°, prior to passage through combustion top dead centre of the following cylinder, as a function of the duration of pilot injection, the other operating parameters being kept constant. This curve presents a minimum plateau. The gradient break point of this curve permits determination of the opening time of the injector from which the injector starts to deliver. This process is intended to be implemented for example at the end of assembly line check to tune the engine or to perform tests in case of malfunction of the engine within the context of after-sales service.
The process described in this document cannot be performed when the engine is outside the idling zone, i.e. when the injectors are being controlled by control signals corresponding to a demand of a gas control organ. This process, which was designed for use during engine idling uses an instantaneous velocity difference which is very sensitive to the shape of the instantaneous velocity curve of the engine at each injection cycle. The present inventors have found that this shape lost its uniformity at high revolutions, so that the difference in question depended as much on the rotational velocity of the engine as on the quantity injected. This resulted in the impossibility of using this process quantitatively other than during idling. Moreover, the difference in instantaneous velocity which is used in this process [is] very dependent on the engine velocity, and this results in a large margin of error if the engine velocity is not constant over the whole learning period.